Field of the Invention
This invention relates to emissive compounds such as those useful in light emitting diodes or devices.
Description of the Related Art
Organic electroluminescent devices capable of emitting white light are desirable because of their potential utility as backplane lights for LCD displays, overhead lighting and other lightweight, low profile, low power lighting applications. White light-emitting Organic Light-Emitting Diode (OLED) devices with high color purity and brightness exceeding 2000 cd/m have been demonstrated by utilizing several fluorescent dyes either doped into one emission layer or segregated into several emission layers. Recently, phosphorescent dyes have been used more frequently as the source of emission in OLEDs because of their 100% maximum theoretical efficiency as compared the theoretical 25% maximum efficiency of fluorescent dyes which emit only from the singlet state. White light emission has been achieved from phosphorescent OLEDs by a number of techniques such as co-doping red, green, and blue phosphors into a single emission layer, and building up a multilayer device with each layer containing a different color phosphor.
A single emissive layer may be desired for ease of device fabrication. However, when multiple dopants are present in the same layer, energy transfer from the high energy dopants to the low energy dopants may be a problem. This may make color balance difficult because the energy transfer may cause more frequent emission from the low energy dopant than from the high energy dopant. In other words, the higher energy dopant may transfer its energy to the low energy dopant instead of emitting the energy as light in the blue range of the visible spectrum. In turn, a greater number of lower energy dopants may emit more energy as light in the red range of the visible spectrum because a greater number of lower energy dopants may be in an excited state due to the energy transfer from the higher energy dopants. Segregation of the various emitters into separate layers may help to overcome the energy transfer problem. However a multilayered device may be more difficult to fabricate and minor changes in layer thickness may result in a significant change in color balance.
One approach to addressing these difficulties has been to prepare a single dopant that can emit white light by the combination of blue emission in its monomer state and orange-red emission from aggregate or excimer species. An excimer is an emissive excited state whose wave function overlaps two adjacent molecules of like composition. One phosphor capable of excimer formation is platinum(II) (2-(4′,6′-difluorophenyl)pyridinato-N,C2′) (2,4-pentanedionato-O,O)6 (FPt). The ratio of monomer/excimer emission is highly concentration dependant with higher concentrations leading to more excimer emission. By careful control of the concentration of FPt, monomer and excimer emission may be balanced and white light may be produced.
On potential problem with FPt is that it may suffer from phase separation or aggregation from electron-transport materials, hole-transport materials, or other host materials which are often helpful to use in an emissive layer of an LED. This may disturb the careful control of the FPt concentration which may be needed to achieve the desired color balance. Thus, there is a continuing need for single-molecule white light emitting phosphors.